With several world leaders still denying climate change and global warming, a 15-year-old has taken it upon herself to school them on its impact through her passionate speeches.

Greta Thunberg, a Swedish climate activist, called out leaders for their inaction during the United Nations climate change summit, COP24, in Poland on Monday and said they were “behaving like children.”

“We will have to take the responsibility they (world leaders) should have taken a long time ago,” she said.

Thunberg addressed a group of people that included UN Secretary Antonio Guterres and pleaded the case for future generations, who have been left to clean up the mess created by previous ones. On Tuesday, Guterres said, “Our younger generations will have to help drive, and complete, the work we start today. We need to harness their energy, invention and political power to raise climate ambition.”

Before capturing the world’s attention from the global platform, Thunberg began a solo protest demanding steps to combat climate change by going on a strike at her school in Sweden in August. The protest soon spread to more than 270 towns and cities in countries like Australia, United Kingdom and the United States, The Guardian reported.